In a resistive circuit Ohm's Law states that voltage is equal to

In a resistive circuit, Ohm's Law states that: voltage is equal to current times resistance.

Ohm's Law: V = I x R

where:

V = voltage (volts)

I = current (amps)

R = resistance (ohms)

1) A circuit consists of a 10 ohm resistor and carries a 5 amp current. What is the

voltage in the circuit (in volts)?

2) One day, Olive has what she thinks is an ingenious idea to connect her dad‟s

ammeter to a flashlight which uses a 6 volt battery. (An ammeter is a device that

measures the current flow, in amps, in an electric circuit.) She connects the

ammeter and sees that there is a current of 3 amps flowing in the circuit. What is

the flashlight‟s resistance?

3) Olive built the circuit below with a variable-voltage power supply (the circuit‟s 1leftmost circuit element) and a resistor defined by the diagram below.

resistor

Using her ammeter (shown by the circuit element with an encircled „A‟), she took measurements of current passing through the resistor. She also used a voltmeter (as shown by the circuit element in the encircled „V‟) to measure the voltage across the resistor. She recorded these numerical values in a table, with the results shown below:

9) A circuit breaker is a safety device that protects a household electric circuit, such as all the electric outlets in a particular room in a house, from having too much electricity running through it. A breaker‟s function is to stop electric current flow

when the current exceeds some preset maximum as given by the breaker‟s “rating”. What do Olive and her twin sister Penny have to do when they both dry their hair at the same time (assume that all of their house‟s bedroom circuit breakers are rated at

15 amps and all bathroom circuit breakers are rated at 20 amps)?

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Answers:

7 a) 250 watts (50 volts x 5 amps)

b) 18 watts (6 volts x 3 amps)

c) 7200 watts (12 volts x 600 amps)

8) a) 0.5 amp (P = I x V ; I = P ? V)

b) 0.1 amp

c) 12.5 amps

9) Since hair driers draw 12.5 amps each (from problem #8), only one hair drier at a time can operate from any single bedroom or bathroom circuit in the house without tripping a circuit breaker. Therefore, if the sisters need to both dry their hair at the same time they either use the same hair drier or they dry their hair in separate rooms.

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Electric “Power Company Law” Practice Problems

Power consumption, which is also known as work, is defined as the total energy used or generated during a given period of time. In electrical circuits it is measured in kilowatt-hours (1000 watts of power lasting for one hour of time).

W = P x t

P - Power consumed by a circuit (typically measured in kilowatts)

t - Amount of time that electric circuit operates

Typically measured in hours for household use.

W - Work, or equivalently: “total power used”

Typically measured in kilowatt-hours (KWH),

but also can be measured in watt-hours, where

1 x KWH = 1000 x watt-hours.

Assume 1 x KWH costs 20? on your monthly electric bill.

Based on the above, you can calculate the cost of energy use for various electrical appliances and equipment if you have these three pieces of information:

The rated power of the appliance, usually given in watts;

The length of operating time,

And the cost of electricity.

10) a) How much power does a 100 watt light bulb use in a year if it is on a timer that

turns on at 6:00 PM every night and off at midnight? (Assume there are

approximately 9,000 hours in a year.)

b) What is the cost of the electricity the bulb consumes in the course of a year?

c) How much would be saved by switching the bulb to a 20 watt CFL bulb (which

produces the same amount of light as the 100 watt bulb)?

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11) a) When their dad pulls out the kitchen refrigerator to clean behind it, Olive sees a

small metal plate on the back of the refrigerator that says it consumes 200 watts of

power. During the cleaning, she notices that the refrigerator comes on for about

four minutes every ten minutes or so. Later that day she tells her twin sister Penny

what she has found out about the refrigerator, and after some figuring, what can

Penny tell Olive about how much it costs to run the refrigerator for a year?

b) Penny sees an on-line ad for a brand new “energy efficient” refrigerator that

costs $1000. The ad claims the refrigerator costs only a dollar a day to run. Should

Penny advise her family to replace their old refrigerator?

12) Penny and Olive have a pet hamster named Max. The twins love to watch Max exercise by running in the spinner in his cage. For a science fair project, Penny got the idea to hook up Max‟s spinner to a fan blade that she disconnected from a handheld battery-operated personal electric fan. After she did so, she thought that when Max was running his very fastest that he was able to turn the fan blade about as fast as the blade had originally spun when attached to the electric fan.

a) The fan could also be plugged into the wall using a power-supply that came with the fan. To help her sister with the science fair project, Olive connected the ammeter and voltmeter to the power supply while the fan was turning the blade and found that the fan operated at three volts and 0.5 amps. How much power did Max expend to turn the fan blade when it was attached to his spinner?

For living things and for nutritional purposes, energy use is usually measured in

Calories, with one nutritional Calorie being approximately equal to 0.001 KWH, or

equivalently, there are approximately 1000 Calories per KWH.

b) If Max always exercised after his dinner every day by jogging for six minutes on

his spinner and assuming he was using one watt of power per second while doing

so, how many Calories did he use to turn the blade during his after-dinner jog?

c) What would be the equivalent electricity cost Max was capable of producing if he

jogged for an hour? How much would this add up to for a year‟s worth of daily